Email open rate is the percentage of delivered emails that recipients actually open. It's primarily a measure of how compelling your subject line and sender name are, since those are all a recipient sees before deciding to open.
Open rate is a useful diagnostic — a low open rate points to weak subject lines or deliverability problems — but it's a vanity metric compared to reply rate. An opened email that goes unanswered hasn't moved you anywhere. In job-search outreach, the only outcome that counts is a response that starts a conversation.
It's also worth noting that open-rate tracking has become less reliable as mail clients pre-load images and privacy features mask opens. Treat open rate as a rough directional signal for your subject lines, and judge your outreach by replies, not opens.